The poem is divided into two parts with two definitely opposite attitudes.  The first part is describing the pain and suffering a sailor goes through living on the sea.  It's "icy bands" chain you to the deck and the skin peels off your hands from the ice and the wind.  It's hard labor, but there are good days, too--when the sea is calm like glass the speaker mentions how far he can see and the birds fly around.  Even though life on the ocean isn't easy, he chooses it over the ease and comfort of life on land.  The sea is in his blood and is part of him.  When he is not at sea, his soul longs to return.  It's a calling.  So the the mood of the first part is one of necessary evil--it's the good and bad of the job the speaker loves described in full detail.  He could not and would not choose any other life.

The second part of the poem is more spiritual in nature.  It becomes almost like a sermon with prayer, and even ends in "Amen".  The speaker explains that there are three ways man will die--old age, illness, or at the hand of another.  Only God knows which our fate will be and a man is a fool who does not fear and obey God.